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DES-7200 Configuration Guide Chapter 8 IGMP Snooping
Configuration
8-4
8.1.4 Understanding Operation Mechanism of IGMP Snooping
8.1.4.1 General Group Query and Specific
Group Query
IGMP querier sends the general query packets to all hosts and routers(with the
address: 224.0.0.1) in the local network segment periodically to query for the IP
multicast group member in the network segment. Upon receiving the IGMP
general query packets, the switch forwards those query packets to all ports in this
VLAN, and processes the packet-receiving port as follows:
If this port has already been in the router port list, reset the aging timer.
If this port has not been in the router port list, add the port to the list and enable
the aging timer.
After receiving the IGMP general query packets, the multicast device enable the
aging timer for all member ports. Set the aging time as the maximum respond
time of the IGMP query packets. When the aging time is 0, no member port
receives the multicast flow and the port will be removed from the IGMP Snooping
forwarding list.
After receiving the IGMP specific-group query packets, the multicast device
enable the aging timer for all member ports in the specific group. Set the aging
time as the maximum respond time of the IGMP query packets. When the aging
time is 0, no member port receives the multicast flow and the port will be removed
from the IGMP Snooping forwardin.
For the IGMP specific-group source query packets, it is no need to update the
aging timer.
8.1.4.2 Membership Report
In the following circumstances, the host sends the IGMP membership report to
the IGMP querier:
After receiving the IGMP query(general or specific-group query) packets, the IP
multicast group member host responds to the received packets.
If the host wants to join in an IP multicast group, it will take the initiative to send
the IGMP membership report to the IGMP querier and claim to join in the IP
multicast group.
Upon receiving the IGMP membership report message, the switch forwards the
message through all router ports in the VLAN, analyzes the IP multicast group
address from the message to add to the host, and deals with the packet-receiving
port as follows:
If the corresponding forwarding entry of IP multicast group is inexistent, create a
forwarding entry, add the dynamic member port to the outgoing port list, and
enable the aging timer.
If the corresponding forwarding entry of IP multicast group exists but the outgoing
port list excludes the port, add the dynamic member port to the outgoing port list,
and enable the aging timer.
If the corresponding forwarding entry of IP multicast group exists and the outgoing
port list includes the port, reset the aging timer.